If you're needing this for a business, or any other form of commerce, I have no knowledge.
If this is a home lab or other non-commercial server though, Microsoft allows you to use an evaluation copy up to almost 3 years. Evaluation copies are good for 180 days, and that counter can be reset in place (no need to reinstall) up to 6 times, for just under 3 years total (1080 days) . I think that for a lot of home lab/tinkerers, 3 years is enough, because there's usually a newer product by then that they can move on to. Copies of almost all the Microsoft Server Products are available for evaluation, but I don't know if all can be reset in this manner for longer term use.
AFAIK this is definitely meant for non-commercial users though. I think that anyone that uses this technique in a commercial environment violates the EULA and opens themselves up to liability issues.